Various means have been used to monitor the operating condition of traveling-wave tubes. A typical technique used is the monitoring of tube body current. When the slow wave structure intercepts the electron beam (a malfunction condition), a current is induced in the slow wave structure. This current flows from the slow wave structure into the body of the tube, and is termed "body current". Although useful, body-current sensing alone does not provide detection of some types of faulty operating conditions which are damaging to traveling-wave tubes. For example, a failure which is not easily detected by body-current sensing alone could be caused by a breakdown of the grid modulator. Such a breakdown may cause the grid-bias to be at zero volts, causing a resultant condition in which the current in the collector is at a potentially destructive level, but where total body current is normal. A similar failure of this nature could be caused by other malfunctions, as for example by an open (i.e. break) in the grid lead. Another failure which most likely would not be detected by body current sensing alone is a condition in which the grid of the traveling wave tube is biased by a d.c. voltage--which causes the body current to be a constant value. This constant value current would appear as zero (not excessive) at a secondary of a current-sensing transformer whose primary winding is in series with the monitored current path.
Additional information as to the malfunction condition of the traveling-wave tube may be provided by sensing and monitoring current in the collector of the tube. In light of the foregoing, and other reasons, it is important to be able to accurately monitor both current in the collector and current in the tube body.
Although collector undercurrent sensing can be helpful in supplying additional information relating to a malfunction condition of the tube, it is not generally used because of inaccuracies inherent in prior monitoring schemes. These schemes tend to give an erroneous indication of the value of the current being monitored, and thereby tend to cause a false indication of tube malfunction condition. Many traveling-wave-tube circuit applications require that collector-current sensing be accomplished within the cathode power-supply portion of the tube circuitry. This necessitates using a high-voltage isolation transformer to sense current flowing in the high-voltage circuit monitored. Since the transformer does not transmit the d.c. level of a current flowing in its primary, it is necessary to restore this level. Only then can the signal from the transformer secondary indicate the true peak value of current being sampled in the transformer primary.
Prior d.c. restorer circuits have utilized various restoring techniques including diode-resistor-capacitor arrangements to accomplish the d.c. restoration. These circuits are objectionable because of their inherent inaccuracy. One cause of this inaccuracy arises in the diode used to pass a negative signal and block a positive one. The diode has an undersired effect resulting in a varying reduction of the restorer output voltage. This variation of the output voltage arises from the fact that current flowing through the diode causes a voltage drop across it, which in turn reduces the voltage at the output. This voltage drop varies with applied voltage, temperature, and other factors which are difficult or costly to compensate for.
The d.c. restorer of the present invention essentially eliminates the error introduced by the diode voltage drop through utilization of a feedback amplifier which allows a capacitor to closely follow the negative-going excursion of the imput signal with virtually no voltage error (e.g., within 2%).
Elimination of the voltage error is especially important when small currents are being sensed by a current-sensing transformer whose output must be d.c. restored. This output is then a low-level voltage signal. Without the accuracy provided by the restorer of this invention, proper sensing of body and collector currents would not only be much more difficult, but erroneous indication of current would tend to result. Furthermore, when a d.c. restorer of the prior-type is used in conjunction with transformer current sampling, the voltage at the output of the transformer must be about an order of magnitude (i.e., ten times) larger than the anticipated restorer voltage error (so that reasonable accuracy in determining the peak current sensed can be maintained). But this requirement for a relatively large transformer output voltage necessitates a relatively large transformer. With a larger transformer, high frequency response becomes degraded--especially when a wide variation in repetition frequency or pulse width of the sampled signal is encountered. This degraded performace tends to limit accuracy of restorer circuits of this nature to the order of about 15% of the current being sensed.
Prior-art d.c. restorers also contain transient errors in their outputs because of charging and discharging of the restorer-capacitor. These transients can result in inaccurate indication of the current sensed. This is especially a problem if a significant voltage overshoot develops as a consequence of rapid change in signal pulse repetition frequency.